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Pluto Reclassified as a Dwarf Planet

Category: Science & Technology Key figures: Mike Brown (co-discoverer of Eris, whose find precipitated the debate); Jocelyn Bell Burnell (chaired the deciding session of the IAU General Assembly)

Summary

On August 24, 2006, members of the International Astronomical Union (IAU), meeting at the 26th General Assembly in Prague, Czech Republic, adopted Resolution 5A, which established the first formal scientific definition of the word “planet.” The resolution defined a planet as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. A separate category, “dwarf planet,” was defined for bodies that satisfy the first two criteria, have not cleared their orbital neighbourhood, and are not satellites.

Pluto satisfied the first two criteria but failed the third, because it shares its region of the Solar System with numerous other Kuiper Belt objects rather than gravitationally dominating its orbital zone. As a result, Pluto was reclassified as a dwarf planet, reducing the number of recognized planets in the Solar System to eight. Ceres and the recently discovered object then designated 2003 UB313 (later named Eris) were named among the first members of the new dwarf-planet category.

The debate was precipitated by the discovery of Eris, found by a team led by Mike Brown, Chad Trujillo, and David Rabinowitz and announced in 2005. Eris was initially measured to be of comparable size to, and slightly more massive than, Pluto, forcing astronomers to choose between admitting additional planets or redefining the term. The vote at the deciding plenary session was conducted by show of cards; Resolution 5A passed by a large majority, and the related Resolution 6A, formally designating Pluto a dwarf planet, passed 237 to 157 with 30 abstentions.

Significance

The 2006 decision marked the first time the scientific community established an explicit, agreed-upon definition of what constitutes a planet, resolving an ambiguity that had persisted since antiquity. By demoting Pluto—classified as the ninth planet since its 1930 discovery—the IAU reduced the Solar System to eight planets and created a formal taxonomy for the growing population of small bodies in the outer Solar System.

The reclassification drew substantial public attention and controversy, with continuing debate among astronomers over the “cleared the neighbourhood” criterion and the relatively small fraction of IAU members who participated in the vote. The decision remains a frequently cited example of how scientific classification evolves in response to new observational evidence, and Pluto’s status continues to be discussed in both scientific and popular contexts.

Sources

• IAU definition of planet — Wikipedia
• IAU 2006 General Assembly: Result of the IAU Resolution votes — International Astronomical Union

Related

• Launch of Twitter — the year’s other landmark Science & Technology development covered in depth.
• North Korea’s First Nuclear Test — the year’s defining geopolitical security event, concurrent with this scientific milestone.
• See the full Timeline of 2006 and master Index for concurrent events.